The present invention relates to image processing techniques and more particularly, to a technique which can be especially effectively applied to a method for compressing and decompressing moving pictures. More particularly, the present invention pertains to a compression/decompression method by which, when it is desired to combine a restored image with another image to form a combined image or video signal for example, a key signal necessary for it can be simultaneously suitably recorded or transmitted together with the combined video signal, and also pertains to a video signal processing system which can remove the key signal from the combined video signal including the key signal to output the combined video signal.
There has been conventionally well known such a method as a xe2x80x9cchroma keyxe2x80x9d scheme, when it is desired to superimpose a moving picture on another image to form a combined image, for image-picking up of a target object with a solid-color or unfigured background behind it and generating a key signal indicative of the presence or absence of the background from the background color for the formation of the combined image.
In such a prior art image combining method as mentioned above, however, when a moving picture is subjected to a high-efficiency coding (i.e., data compression) and then to a decompressing operation, the compressed moving picture cannot always be restored to the original image and thus the key signal cannot sometimes be reproduced correctly. For this reason, a problem sometimes occurs that such a place (which will be referred to as the background, hereinafter) that is to be replaced by another image on the basis of the key signal cannot be correctly replaced thereby or conversely such a place (which will be referred to as the foreground, hereinafter) that is not to be replaced by another image can be wrongly replaced thereby.
It is an object of the present invention to provide a method for compressing and decompressing moving pictures on a high-efficient coding basis which can eliminate the above problem in the prior art and can reproduce a key signal correctly.
Another object of the present invention is to provide a video signal processing circuit which can reproduce a key signal highly accurately and can decompress a moving picture subjected to a high-efficiency coding to reproduce a high quality of original image.
In accordance with an aspect of the present invention, to achieve the above objects there are provided a method and system for compressing and decompressing a moving picture which includes a step or means for performing conversion to separate values of an original video signal from a value of a key signal indicative of background or non-background and a step or means for performing reverse conversion opposite to the coding mode, and wherein the separation between the video signal and key signal from the compressed and decompressed image is carried out with use of a threshold for discrimination.
In another aspect of the invention, it is desirable that key signals of both pixels adjacent to a target pixel of reproduction video information are referred to and when either one of the key signals indicates background, the key signal of the current pixel is changed to have a value indicative of background.
With the above arrangement, when the range of the key signal and the video signal be uniquely specified to make large the distance in signal level between the ordinary video signal and the key signal and to perform coding thereover, the separation between the video and key signals from the compressed and decompressed image can be facilitated and thus the video and key signals can be correctly reproduced. Further, since the discrimination between both signals is carried out with use of the threshold, the correct separation between the video and key signals can be carried out while avoiding an error in the decompressed video signal. Furthermore, since the threshold is prepared to have a plurality of values, a more accurate separation between the video and key signals can be realized. In addition, since key signals of both pixels adjacent to a target pixel of the reproduction video information are referred to and when either one of the key signals indicates background, the key signal of the current pixel is changed to have a value indicative of background, and the reproduction of the key signal can be realized at the border between the background and the foreground.